For the guide wire for a catheter used for interposing into a human body, etc., it is required to be interposed easily making the tip portion of wire material thin and flexible by tapering. As an example, a portion of 100 to 200 mm at the tip of wire material of around 0.5 mm.phi. is tapered so that the diameter of the utmost tip portion is around 0.12 mm.phi. is used.
Traditionally, this tapering has been made first by mechanical polishing, but the mechanical polishing has a drawback of losing the superelasticity for the Ni-Ti superelastic alloy used usually as a guide wire for a catheter. Recently, therefore, an electrolytic polishing method by batch process is adopted, wherein the tip portion of a wire material is dipped, for example , into a polishing solution having a composition comprising 92% of CH.sub.3 COOH and 8% of HClO.sub.4, into a polishing solution having a composition comprising 20% of HF and 80% of HNO.sub.3, or into a polishing solution having a composition comprising 60% of H.sub.2 SO.sub.4, 30% of HF and 10% of glycerine.
This traditional electrolytic polishing method is however a method of polishing batch wise by dipping one by one into the electrolytic polishing bath while moving up and down manually. Hence, there arise the problems that the shape of the tapered portion varies widely and many workers are required, thus resulting in a high cost.
Moreover, traditional electrolytic polishing baths have the following problems. Namely, the acetic acid-perchloric acid bath has a strong bad smell and gives somewhat poor surface gloss after polishing, and the fluoric acid-nitric acid bath has a strong bad smell, gives poor surface gloss after polishing and requires high cost for effluent treatment to avoid a pollution problem. The sulfuric acid-fluoric acid-glycerine bath has no bad smell, but it gives poor surface gloss and requires high cost for effluent treatment.